## Saturday, December 3, 2011

### Coefficient of Friction

By Julie Rahm

This week, my husband, John was stepping off our boat when his cell phone fell into the water. Even more unfortunate; the cell phone was in his pocket! You see, the steps were wet from the morning dew. When his front foot touched down on the wet plastic steps, his slick bottomed, four-year-old Crocs failed to provide an adequate coefficient of friction. His fall into the water allowed me to use my University of Nebraska physics degree one more time.

The static friction coefficient between two solid surfaces is defined as the ratio of the tangential force required to produce sliding divided by the normal force between the surfaces. Clearly, John has not studied enough physics! His front foot slipped forward. John attempted to regain his balance by getting his back foot down onto something solid. Unfortunately, he stepped down onto the thin air between the boat and pier. At this exact moment, the coefficient of friction gave way to gravity. And, another opportunity for physics!

Newton's law of universal gravitation states that every point mass in the universe attracts every other point mass with a force that is directly proportional to the product of their masses and inversely proportional to the square of the distance between them. A simple enough concept from 1687! Had John studied physics instead of PhysEd, he would still have a cell phone! Regardless, he hurled toward the water accelerating at 32 feet-per-second squared until his chest caught the edge of the pier. As you remember, Newton’s first law states the velocity of a body remains constant unless the body is acted upon by an external force. And, John’s body was definitely acted upon by the static force of the pier! One good bounce and splash, he was in the water and swimming, with his cell phone in his pocket.

As soon as John hit the water, the second law of thermodynamics kicked in. The law asserts that when two isolated systems in separate but nearby regions of space, each in thermodynamic equilibrium in itself, are at some time allowed to interact allowing them to exchange matter or energy, they will eventually reach a mutual thermodynamic equilibrium. You see, when John hit the water, the two systems interacted. John and the water were going to reach thermodynamic equilibrium. Simply, John was going to freeze if he didn’t get out of the water and dry off quickly. He confirmed the second thermodynamic law by uttering, “Wow, the water is cold.” John swam to our dingy platform and pulled himself out of the cold water.

John is no worse for the experience. A skinned shin, a bruised wrist, bruised ribs and loss of a cell phone are minor compared to the other possible outcomes. Life can unexpectedly turn on a dime. So, I encourage everyone to savor the moments of their lives. Enjoy and care for your blessings. Because, in a splash, they can be gone. Can you tell I've been watching late night Big Bang Theory re-runs? Visit my websites at http://http://www.themindsetmechanic.com/ and http://www.militarykidsspeak.com/ for more inspiration!

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